Linear accelerators, or Linacs, are devices which use radio frequency energy to accelerate charged particles. In such devices charged particles from a source are passed through a series of drift tubes which are separated from one another by gaps. A potential difference across the gaps, supplied by the radio frequency energy, is used to accelerate the particles.
FIG. 1 indicates a basic prior art linear accelerator system wherein charged particles are generated from an ion source injector 10. Injected charged particles enter the accelerator section of a linear accelerator 12 for the purpose of greatly increasing the velocity of the charged particles. The linear accelerator 12 often includes a hollow cylindrical structure known as a drift tube tank 13. The axis of the drift tube tank 13 is co-linear with the injected beam of charged particles.
A number of drift tubes 18 are arranged in the tank 13, the drift tube body being coaxial with the tank axis. The phase of the radio frequency voltage within the tank provided by the R.F. source 14 and feed 16 is such that the particles are accelerated toward the tank exit 22. Each time particles are accelerated across a gap existing between adjacent drift tube bodies, they momentarily enter a successively positioned drift tube, where they become sheltered from the effects of reversals of the oscillating R.F. voltage. As the particles emerge from each of the drift tubes, the phase of the radio frequency voltage is such as to accelerate the particles toward the next succeeding drift tube. This process is repeated again between drift tubes to achieve the desired particle energy. It should be noted that the lengths of the drift tube bodies increase as necessary to compensate for the increasing velocity of the particles so that the time required for the particles to travel between adjacent drift tubes is always one period of the RF voltage. The finally accelerated charged particles exit the linear accelerator 12 at 22 and become directed at a target 24.
Because of interaction the accelerating charged particles heat the drift tubes 18, it is necessary to introduce coolant into the drift tubes. This is achieved by the stems being hollowed so that coolant may be provided by pipes 25 connected to a coolant reservoir 23.
In the prior art means were provided to initially accelerate the charged particles so that they would enter the linear accelerator at the designed injection velocity. Typically, an accelerator of known type, such as a Cockcrost-Walton, was used as a pre-accelerator. However, understanding of the present invention does not require a description of prior art pre-accelerators, except to note that such pre-accelerators were expensive and complicated. Buncher means to bunch the particles so that they would enter the linear accelerator at proper phase of the RF voltage were also known in the prior art. It should be noted that linear accelerators are provided with a vacuum system for maintaining the vacuum necessary for the acceleration of the charged particle beam.
Although FIG. 1 illustrates a simplified linear accelerator as including only three drift tubes, it should be understood that a large number of such drift tubes are necessary. In the prior art, grooves were machined in tube components to serve as coolant conduits within a drift tube. Inlet and outlet ports were also machined in each drift tube to permit the circulation of coolant flow through the drift tube. Individual components forming a drift tube were then brazed or welded together to form a completed unit. The great disadvantage resides in the high cost involved in machining and brazing operations. Further, an inherent disadvantage exists when individual components of a drift tube are brazed together. This is due to the fact that a number of interfaces are formed, each of which represents a potential point of coolant leakage.